(1) Field
The present invention relates, in general terms, to piezoelectric vibrating gyrometers which make it possible for the rotation of an object in space to be measured.
The invention relates more especially to piezoelectric excitation and detection electrodes for the two useful modes of a tuning fork and to associated electronic circuits, which make it possible to avoid coupling, by an electrostatic effect, between the electrodes dedicated to one of the two useful modes and those dedicated to the other useful mode, while providing good piezoelectric efficiency of those electrodes and admitting of a low cost of manufacture.
(2) Description of the Related Art
The principle of operation of a vibrating gyrometer is based on detection of Coriolis accelerations which act on a vibrating resonator in accordance with a first useful mode referred to as the “drive mode”, when this resonator is subjected to an angular rate of rotation {right arrow over (Ω)} referenced relative to an inertial reference frame referred to as the “Galilean reference frame”. Accordingly, the Coriolis accelerations are alternating at the frequency of said first useful mode and excite a second useful vibration mode referred to as the “detection mode”, the amplitude of the vibration of which is proportional to Ω. Measurement of this amplitude, generally converted into the form of electrical signals, accordingly makes it possible to determine Ω.
It is known that the vibrating element may be a tuning fork having two tines attached to a common part, the drive mode of which is a flexural vibration of the two tines in mutual phase opposition parallel to the plane of the tuning fork and the detection mode of which is a flexural vibration of the two tines in mutual phase opposition perpendicular to the plane of the tuning fork.
Accordingly it is known for the tuning fork to be produced in a plane plate of quartz, the faces of which are parallel to the crystallographic plane XY, the tines of the tuning fork being oriented parallel to the Y axis, and for means for maintaining the drive mode in vibration and detecting the detection mode to be electrodes in the form of ribbon conductors disposed longitudinally on the two tines and acting by a piezoelectric effect.
More generally, it is known to the person skilled in the art that other piezoelectric crystals and other orientations of the plate in the crystallographic reference frame are possible, the important aspect being to obtain sufficient piezoelectric coupling between extension or compression of the longitudinal fibres of the flexurally vibrating tines and the electric field or displacement of the charges in directions perpendicular to the longitudinal axes of the tines.
The electrodes for maintaining the drive mode in vibration are connected to an electronic oscillator circuit which makes it possible to maintain the drive mode in resonance vibration and comprise at least one excitation electrode of the drive mode on which there is present an alternating excitation potential and at least one detection electrode of the drive mode, in order to make possible the electromechanical oscillation of the drive mode. The value of working in resonance vibration lies in reconciling a large amplitude of vibration of the drive mode, and therefore good sensitivity of the gyrometer, with low electrical consumption of the device. In order to facilitate the description, said electrodes for maintaining the drive mode in vibration will be referred to, in their respective cases, as the drive excitation electrode and drive detection electrode. The electrodes for detecting the detection mode are connected to an electronic detection circuit which makes it possible to produce an electric voltage representative of the rotation rate to be measured. Unlike the drive mode, the detection mode generally works in forced vibration and therefore requires only one detection electrode, referred to as the Coriolis detection electrode.
It is also known for drive excitation, drive detection and Coriolis detection electrodes to cohabit side-by-side along each of the two tines, especially in the vicinity of the end attached to the common part, because that is where the mechanical stresses of the flexural vibrations are maximal, that is to say where the piezoelectric effect is maximal. This makes it possible to obtain good piezoelectric efficiency for the totality of the electrodes and therefore relatively low drive excitation electric consumption and relatively high Coriolis detection sensitivity.
The side-by-side disposition of the drive excitation, drive detection and Coriolis detection electrodes makes it necessary to take certain precautionary measures in order to avoid the alternating electric potentials for excitation of the drive mode giving rise, by the electrostatic effect, to the appearance of undesirable electrical signals on the Coriolis detection electrodes, it being possible that such signals might be falsely interpreted as being caused by rotation at a rate Ω applied to the device. It is accordingly known (U.S. Pat. No. 5,939,631) to employ two means which are cumulative in their efficiencies.
The first known means consists of applying two alternating electric potentials in phase opposition to two drive excitation electrodes respectively, which may be obtained by means of an arrangement of the oscillator circuit, and having the totality of the electrodes disposed in such a way that, as a result of symmetry, the Coriolis detection electrodes each receive, by the electrostatic effect, the same amount of positive undesirable signals as negative undesirable signals, that is to say they each receive an undesirable signal which overall is zero. In practice, in view of usual production imperfections, this first known means makes it possible to reduce the amplitude of the undesirable signals by one to two orders of magnitude.
The second known means consists of interposing between the drive excitation electrodes and the Coriolis detection electrodes a ground screen in the form of additional ribbon conductors disposed on the two tines of the tuning fork and connected to the electric ground of the device. These additional ribbon conductors have the same general appearance as electrodes and because of that they cannot form a total screen between the drive excitation electrodes and the Coriolis detection electrodes but they nevertheless make it possible to reduce the amplitude of the undesirable signals by a further one to two orders of magnitude, that reduction being added to that obtained by the first known means.
It must be emphasised that the simultaneous implementation of those two known means, which makes it possible in practice to reduce the amplitude of the undesirable signals by three to four orders of magnitude, is very useful in view of the very low values of the signal produced by the Coriolis accelerations. Accordingly, by way of example, an angular rate of rotation of 1 degree/second can generate, by the piezoelectric effect, at the Coriolis detection electrodes, electric charges of the order of 10−16 Coulomb, whereas the undesirable charges generated by the electrostatic effect can be of the order of 10−12 Coulomb before implementation of the two known means. Under those conditions, the two known means make it possible to reduce the undesirable charges to values of the order of 10−16 to 10−15 Coulomb, which is sufficiently low for signal processing operations to make it possible to obtain a gyrometer measurement precision of the order of 0.01 degree/second.
In the case of the known arrangement described in the above-mentioned patent specification, the addition of additional ribbon conductors connected to the electric ground of the device has few drawbacks, because the electrodes are disposed on the four longitudinal faces of each of the tines and because of that there is sufficient space for the presence of the additional ribbons not to encroach significantly upon the space available for the electrodes and therefore practically not to adversely affect the piezoelectric efficiency of the electrodes. It will be noted that the tuning fork of the known arrangement is obtained by chemical forming of a quartz plate, that the electrodes and additional ribbons can be obtained by means of photolithographic processes used in the low-cost manufacture of clock resonators, but that producing the electrodes disposed on the formed sides of the tines requires substantially more onerous means such as evaporation of metal through a mechanical mask disposed on the formed tuning fork.
In the case of low-cost manufacture, that is to say where it is necessary to rule out placing electrodes on the formed sides of the tines and therefore to make do with the faces parallel to the plane of the plate, the addition of the additional ribbons connected to the electric ground of the device can be accomplished only at the expense of the space available for the electrodes and therefore of the piezoelectric efficiency of the electrodes.
Accordingly, the teaching of the prior art does not make it possible, in the case of low-cost production, to benefit simultaneously from a substantial reduction in the undesirable signal and from satisfactory piezoelectric efficiency of the electrodes.